1. Field of the Invention
The invention relates to a method for approximating an influence of an optical system on the state of polarisation of optical radiation. Such a method can e.g. be applied to microlithographic systems. The term “optical system” is to be understood as any arrangement of one or more optical components which transmit and/or reflect incident optical radiation, in particular including lenses and objectives constructed therewith. In the case of a microlithographic system the optical system can e.g. be the projection objective of such a system. The term “optical radiation” is to be understood as any desired electromagnetic radiation which is applied to the optical system under test, for example visible light or UV radiation. In order to describe the state of polarisation and how it is influenced or changed by the optical system, use is generally made of suitable variables such as the Stokes parameters, the Muller matrix, the Polarisation matrix and the Jones matrix. Reference may be made to the relevant literature for details in this regard.
2. Description of the Related Art
A method for determining an influence of an optical system on the state of polarisation of optical radiation is described in the German patent application 103 04 822 A1. With this method a Jones matrix of the optical system is determined in two measurement stages. In the first stage entrance-side radiation of defined polarisation states is directed onto the optical system. The intensities of the resulting exit states of polarisation of the radiation emerging from the optical system is then measured using a polarisation analyser. From this measurement data a phase-reduced Jones matrix is calculated. In a second measurement stage a global pupil resolved phase term is determined by an interferometric measurement. In this measurement one single entrance-side polarisation state is used for determining the global phase term of the resulting radiation emerging from the optical system. The phase-reduced Jones matrix determined in the first measurement stage and the global phase term is then combined in order to obtain the complete Jones matrix of the optical system.
In the patent application U.S. 2002/0024673 A1 a wave front measuring device is described for obtaining information on the influence of an optical system on the state of polarisation of optical radiation. This wave front measuring device contains an entrance-side polariser for generating at least two different linearly polarised states, an interferometer of the Twyman-Green or Fizeau type for measuring a wave front. Further, the wave front measuring device includes a detector-side polariser, which is required to analyze the polarisation state of the light emanating from the optical system.